...and yet another data point...
...on my -6A, using Van's 60A internally regulated alternator, and the recommended belt, I found that I could only install the belt if...I removed the ring gear, slipped on the belt, strrrrrrrtched the belt over the alternator pulley, and then reassembled. So, even if a spare belt was somehow placed behind the prop out of the way...I still would have to remove the prop and ring gear to get the new belt installed.
If I had used the next size longer belt, then the alternator pulley would have hit the lower fiberglass cowling, so that was not an option.
In the old days of typewriter repair (IBM Selectric), it was common for the field guys to install a second, spare motor-drive tooth belt inside the typewriter, loosely around the drive shaft clutch. This labor saving procedure was beneficial when the first belt wore out, and the spare belt could be slipped in place in just a few moments (the replacement of the first belt...the first time...was on the order of 30min). About halfway through the life production run of these typewriters, the belt material chemistry was changed and it was found that the life of these drive belts had lengthened to the point that if a spare belt was provided, it did not get used because service life environment made it no better that the belt that had worn out.
For me, it looks like installing a new, fresh belt is the way to go.